The present invention relates to a vacuum control valve for automatically varying value of negative pressure from a vacuum source such as a vacuum pump. The invention relates in particular to a vacuum control valve used with an exhaust gas recirculation valve (hereinafter referred to as EGR valve) and for controlling the negative pressure of the vacuum applied to the EGR valve.
Recently it has become necessary to purify exhaust gases from Diesel engines and many experiments have been made for the purpose. One of the difficulties for Diesel engines is that a vacuum source, such as an intake negative pressure representing an operational load of an engine, is not present in Diesel engines. Therefore it is necessary for Diesel engines to provide a vacuum pump driven by the engine and a vacuum control valve for modulating the negative pressure from the vacuum pump in order to produce such a negative pressure as representing an operational load of the engine.
Such a vacuum control valve is proposed by some of the present inventors in a U.S. patent application of Ser. No. 370,069 filed on Apr. 20, 1982. In this vacuum control valve, a preload of a spring is changed in response to a rotational angle of a cam member in order to change the negative output pressure, and therefore the amount of the pressure change depends on a spring constant and a slope of the cam member. When it is required to this vacuum control valve to change the output pressure rapidly at a predetermined rotational angle of the cam member, it is necessary to provide another equipment such as a vacuum switch, an electromagnetic solenoid, etc. Otherewise it is necessary to make the slope of the cam member to change rapidly. However, when the slope of the cam member is rapidly changed, a smooth movement of a cam follower is prevented.